This invention relates generally to a method and apparatus for applying coatings to fasteners having internal bores. More particularly, the present invention relates to a method and apparatus for applying powdered coating materials to portions of the internal bore of a fastener or similar article that is open at at least one end, primarily utilizing centrifugal force, rather than an airstream to propel the coating material towards the surface of the fastener in order to form a 360.degree. coating thereon.
Various methods and apparatus have been disclosed in the prior art for applying powder coatings to articles such as fasteners. Most of these efforts have been directed towards the application of coatings to fasteners having an external threaded surface. Since the threads of the fastener desired to be coated are in such instances are completely exposed, they do not pose the increased difficulty that is present when it is desired to provide coatings on fasteners or threaded articles that have internal bores or threads. The existing solutions to providing a 360.degree. coating on the internal threads or an internal bore of a fastener have to date been cumbersome and inefficient, resulting in inconsistencies and increased production costs. In most of these prior devices, the internally threaded fasteners to be coated are first heated and then a nozzle is inserted into the threaded opening, which delivers powder particles entrained in an airstream which fuse and coalesce they contact the heated fastener threads. Typical of such systems is the apparatus disclosed in U.S. Pat. No. 4,835,819. In that device, a significant air pressure is required to be induced through a network of spider-like tubes and ultimately issuing through small nozzles at the end of the tubes directing coating material toward the threads of the fastener. The generating of an airstream under significant pressure required by such systems is both costly and difficult to regulate. Having to split the generated airstream equally into multiple tubes likewise adds problems. This is particularly true given the small sized diameters of the tube and openings of the nozzle when small internally threaded articles are being processed. The device also requires the powder to change direction multiple times during its travel in the airstream through the tube and the nozzle. Again, given the dimensions, these systems have been susceptible to regular clogging of tubes or nozzles, as well as inconsistent powder flow. The force generated by the airstream against the inner walls of the fastener is significant and requires the entire outer surface of the fastener to be surrounded by a fixture in order to prevent horizontal movement at the fastener during processing due to this force.
Several other types of methods and apparatus for forming 360.degree. coatings on internal bores or threads of fasteners have also traditionally been utilized. For example, U.S. Pat. No. 4,865,881 discloses an apparatus and process for making locking slide nuts. In this device, a fastener opening is filled with locking material in an amount significantly greater than the amount required to form the coating on the threads. A non-rotating clearance pin is inserted into the opening to attempt to direct the material towards the area of the fastener adopted for internal threading prior to heating and remains in that position while the fastener is heated and the coating material hopefully adheres to the inner walls of the fastener. The clearance pin may then have to be used selectively to clear a passage way through the locking material, either before or after the heating step. In this device, although the clearance pin serves to deflect some of the powder towards the walls of the internal opening of the fastener, it does so with insufficient force to maintain a significant amount of that powder against the walls. In addition, any vertical motion of the clearance pin after the coating has been formed can easily dislodge the entire coating from the desired area of the internal opening of the fastener.
U.S. Pat. No. 4,891,244 describes a method and apparatus for making self-locking fasteners utilizing a mechanical propelling device which comprises a rotatable slinger. The slinger propels particles by centrifugal force against heated threaded surfaces of fasteners. This device, however, only contemplates the coating of threaded fasteners over a circumference of 180.degree. or less. This system further requires that the powder be fed to and confined in four small diameter tubes at different vertical heights that are spaced circumferentially every 90.degree. along the disc in order to be discharged toward the fastener surfaces. The facetless surface requires the disc to be a large diameter, in order to accelerate the powder particles to a velocity which will spray horizontally over the significant distance from the slinger to the bolt surface. The increased velocity imparted to the powder particles causes the vast majority of powder to bypass or bounce off the fastener. This has contributed to inconsistent powder flow and coating results.
In addition to the shortcomings set forth above with respect to the prior art devices, none of these devices had the ability to apply two different powdered coating materials to a single fastener during the coating process. Prior known systems also fed significantly more powdered coating material toward the threads than ultimately ended up coalescing and forming the coating. This increased the frequency of powder flow problems when this excess powder was collected and ultimately recirculated. It is apparent, therefore, that there is a need to be able to form 360.degree. coatings on articles such as fasteners that have internal bores open at at least one end or threads, without the necessity of entraining the powdered coating material in a pressurized airstream.